This disclosure concerns an invention relating generally to reduction of emissions such as particulates and NOx in internal combustion engines, and more specifically to emissions reduction in compression ignition (CI or diesel) engines.
Common pollutants arising from the use of internal combustion engines are nitrogen oxides (commonly denoted NOx) and particulates (also known simply as xe2x80x9csootxe2x80x9d). NOx is generally associated with high-temperature engine conditions, and may be reduced by use of measures such as exhaust gas recirculation (EGR), wherein the engine intake air is diluted with relatively inert exhaust gas (generally after cooling the exhaust gas). This reduces the oxygen in the combustion region and obtains a reduction in maximum combustion temperature, thereby deterring NOx formation. Particulates include a variety of matter such as elemental carbon, heavy hydrocarbons, hydrated sulfuric acid, and other large molecules, and are generally associated with incomplete combustion. Particulates can be reduced by increasing combustion and/or exhaust temperatures, or by providing more oxygen to promote oxidation of the soot particles. Unfortunately, measures which reduce NOx tend to increase particulate emissions, and measures which reduce particulates tend to increase NOx emissions, resulting in what is often termed the xe2x80x9csoot-NOx tradeoffxe2x80x9d.
At the time of this writing, the diesel engine industry is facing stringent emissions legislation in the United States, and is struggling to find methods to meet government-imposed NOx and soot targets for the years 2002-2004 and even more strict standards to be phased in starting in 2007. One measure under consideration is use of exhaust after-treatment (e.g., particulate traps) for soot emissions control in both heavy-duty truck and automotive diesel engines. However, in order to meet mandated durability standards (e.g., 50,000 to 100,000 miles), the soot trap must be periodically regenerated (the trapped soot must be periodically re-burned). This requires considerable expense and complexity, since typically additional fuel must be mixed and ignited in the exhaust stream in order to oxidize the accumulated particulate deposits.
Apart from studies directed to after-treatment, there has also been intense interest in the more fundamental issue of how to reduce NOx and particulates generation from the combustion process and thereby obtain cleaner xe2x80x9cengine outxe2x80x9d emissions (i.e., emissions directly exiting the engine, prior to exhaust after-treatment or similar measures). Studies in this area relate to shaping combustion chambers, timing the fuel injection, tailoring the injection rate during injection so as to meet desired emissions standards, or modifying the mode of injection (e.g, modifying the injection spray pattern). One field of study relates to premixing methodologies, wherein the object is to attain more complete mixing of fuel and air in order to simultaneously reduce soot and NOx emissions. In diesel engines, the object of premixing methodologies is to move away from the diffusion burning mechanism which drives diesel combustion, and instead attempt to attain premixed burning. In diffusion burning, the oxidant (fuel) is provided to the oxidizer (air) with mixing and combustion occurring simultaneously. The fuel droplets within an injected spray plume have an outer reaction zone surrounding a fuel core which diminishes in size as it is consumed, and high soot production occurs at the high-temperature, fuel-rich spray core. In contrast, premixed burning mixes fuel and air prior to burning, and the more thorough mixing results in less soot production. Premixing may be performed by a number of different measures, such as by use of fumigation (injection of fuel into the intake airstream prior to its entry into the engine), and/or direct injection of a fuel charge relatively far before top dead center so that piston motion and convection within the cylinder result in greater mixing.
One promising diesel premixing technology is HCCI (Homogeneous Charge Compression Ignition), which has the objective of causing initial ignition of a lean, highly premixed air-fuel mixture near top dead center (near the end of the compression stroke or the beginning of the power stroke). An extensive discussion on HCCI and similar premixing techniques is provided in U.S. Pat. No. 6,230,683 to zur Loye et al., and U.S. Pat. No. 5,832,880 to Dickey and U.S. Pat. No. 6,213,086 to Chmela et al. also contain useful background information. The charge is thus said to be xe2x80x9chomogeneousxe2x80x9d in HCCI because it is (at least theoretically) highly and evenly mixed with the air in the cylinder. Ignition is then initiated by autoignition, i.e., thermodynamic ignition via compression heating. The objective is to use autoignition of the lean mix to provide significantly lower combustion chamber temperatures, thereby diminishing NOx production (which thrives at high temperature). In contrast, a richer mixture (such as that necessary for flame propagation from the spark in an SI engine) will burn more quickly at greater temperature, and therefore may result in greater NOx production.
As the foregoing references note, while the HCCI process might be beneficially implemented, it is also hard to accomplish owing to the difficulties in igniting the lean mix and/or controlling the start of ignition. Combustion in an SI engine is readily initiated by the spark, with premixed burning occurring afterward; similarly, combustion in a conventional CI engine is initiated by fuel injection near top dead center (following compression) when thermodynamic conditions for autoignition are favorable, with diffusion burning occurring afterward. However, HCCI does not utilize a spark, nor is it desirable to use the rich mixture needed for effective use of a spark. It is also difficult for HCCI to achieve a homogeneous charge or premixed burning if injection near top dead center (during or after compression) is used, since there is less time for mixing to occur. Thus, a key area of study in the HCCI field is how and when to efficiently initiate ignition, and ignition and timing problems are the primary reason why HCCI has not attained widespread use.
Multiple injection, also called split injection, pilot injection, and post injection, has also been a proposed method for NOx and particulate emissions reduction in diesel engines (see, e.g., Tow, T., Pierpont, A. and Reitz, R. D. xe2x80x9cReducing Particulates and NOx Emissions by Using Multiple Injections in a Heavy Duty 0.1. Diesel Engine, xe2x80x9d SAE Paper 940897, SAE Transactions, Vol. 103, Section 3, Journal of Engines, pp. 1403-1417, 1994). A multiple injection engine varies from the standard xe2x80x9csingle injectionxe2x80x9d engine in that the direct injection of a single fuel charge during the combustion cycle is replaced by direct injection of several fuel charges spaced over time, with less fuel being used per injection so that the total amount of fuel finally injected per cycle is comparable to that used in single injection. The multiple injections take place around top dead center (after compression), and burning occurs in a diffusion mode wherein each charge burns upon injection, without premixing. Thus, the division of the xe2x80x9cstandardxe2x80x9d single injected charge into several smaller discrete charges spaced over time results in steady and more complete burning of the injected fuel plumes with more evenly maintained combustion temperature, which helps decrease emissions. While multiple injection is not in common use at the time of this writing, engines using the multiple injection concept are now in production or under development in Europe, Japan and the United States.
Further, while multiple injection will assist the diesel engine industry in meeting emissions goals, it unfortunately does not appear to be a complete solution: it does not by itself decrease emissions to the minimum levels desired. There is thus a significant need for methods and apparata which assist in compression ignition or diesel engine emissions reduction.
The invention involves a premixing methodology which is intended to at least partially solve the aforementioned problems. To give the reader a basic understanding of some of the advantageous features of the invention, following is a brief summary of preferred versions. As this is merely a summary, it should be understood that more details regarding the preferred versions may be found in the Detailed Description set forth elsewhere in this document. The claims set forth at the end of this document then define the various versions of the invention in which exclusive rights are secured.
During a combustion cycle, a first stoichiometrically lean fuel charge is injected long prior to the intended time of ignition, preferably at any time between top dead center and bottom dead center during the intake stroke, or after bottom dead center and early in the compression stroke. Injection of the first fuel charge during the intake stroke is particularly preferred. As the piston progresses towards bottom dead center, the motion of the piston and the cylinder gases provides a high degree of mixing of the first fuel charge and the air within the cylinder, resulting in a more homogeneous fuel/air mixture.
Prior to the time when ignition is desired, a subsequent fuel charge is injected to create a stratified, locally richer mixture (but still leaner than stoichiometric). While this injection may occur at almost any time during the compression stroke depending on speed and load conditions (generally between a time shortly after bottom dead center and a time shortly prior to top dead center), later injection is more desirable since it provides lesser mixing time and motion, thereby enhancing stratification. The locally rich region within the combustion chamber has sufficient fuel density to autoignite (for example, it may constitute the remaining 50-20% of the xe2x80x9cstandardxe2x80x9d charge), and its self-ignition serves to activate ignition for the lean mixture existing within the remainder of the combustion chamber. Because of the use of HCCI combustion conditions within the major portion of the combustion chamberxe2x80x94i.e., a premixed and overall relatively lean mixturexe2x80x94NOx and soot are significantly diminished.
It can be appreciated that the invention is more than an evident extension of HCCI concepts. Common HCCI methods seek to attain an extremely high degree of premixing, with the objective of producing an entirely homogeneous charge within the combustion chamber prior to the desired time of ignition (which is difficult to control, as previously noted). In contrast, while the present invention seeks to attain some degree of premixing with the first charge (with this charge being desirably, though not necessarily, homogeneously dispersed throughout the combustion chamber), the later injection largely obviates homogeneity by creating a highly stratified, locally richer ignition region within the combustion chamber. It can also be appreciated that while the invention utilizes the concept of multiple injection, it does not use it for the purpose for which multiple injections are generally intended. Standard diesel multiple injection methodologies have an initial fuel charge injected and ignited at or slightly before top dead center during the compression stroke, and then follow with subsequent injections spaced over time to maintain a controlled combustion rate. Combustion occurs via a diffusion burning mechanism, whereby each injection is burned at the time of injection. In contrast, the first injection in the present invention is done relatively far before top dead center during compression (and is not immediately ignited), and a subsequent injection is done shortly prior to top dead center during compression, with the objective of initiating premixed burning throughout the entirety of the combustion chamber. Thus, the invention is more than a simple amalgamation of HCCI and multiple injection concepts.